Python implementation of DCP (FastAPI & HTTPX support)

[MatteoScorcia]

This PR provides a modular Python implementation of the Differential Context Protocol (DCP), optimized for both server-side streaming and client-side consumption.

Following previous feedback, the architecture has been refined to decouple networking dependencies (FastAPI/HTTPX) into optional extras, ensuring the package remains lean and suitable for diverse environments (from
high-performance servers to lightweight background workers).

Core Features:

• Modular Architecture: Split into [server] (FastAPI), [client] (HTTPX), and core logic (Pydantic-only) to minimize dependency overhead.
• FastAPI / Starlette Integration: ObjectStreamResponse for turning async generators into streaming endpoints with automatic diffing.
• Resilient Async Client: fetch_object_stream for real-time state reconstruction, featuring heartbeat support and automatic fallback to plain JSON.
• Optimized Diffs: High-performance calculation of deltas using JSON Pointers (RFC 6901) and specialized string-append operations.
• Dictionary-based Path Compression: 95%+ compression efficiency during transmission for redundant JSON paths.
• Legacy Compatibility: Supports Python 3.10+.

[MatteoScorcia]

I have completely refactored the original Python implementation to address the modularity concerns. The package now features a clean import tree, supports Python 3.10+, and cleanly separates the server and client logic into optional extras while maintaining the full feature set of the DCP specification.